Conventionally, dyes have been widely used in various technical fields. As one example, in the field of photoelectric conversion elements, for example, a dye having photosensitization action is used in the working electrode of a dye-sensitized solar cell.
A dye-sensitized solar cell generally has an electrode having an oxide semiconductor as a support for a dye. Such a dye absorbs incident light and is excited, and this excited dye injects electrons into the support to perform photoelectric conversion. In this type of dye-sensitized solar cell, high energy conversion efficiency can be theoretically expected among organic solar cells. In addition, this type of dye-sensitized solar cell can be produced at lower cost than conventional solar cells using a silicon semiconductor and therefore is considered to be very advantageous in terms of cost.
On the other hand, as dyes used in photoelectric conversion elements, organic dyes, such as ruthenium complex dyes and cyanine dyes, are widely known. Particularly, cyanine dyes have relatively high stability and can easily be synthesized, and therefore, various studies have been made.
For example, Patent Document 1 discloses a cyanine dye that has a structure in which an indolenine skeleton is bonded to both ends of a methine chain skeleton, and further has a carboxylic acid group as an anchor group to be adsorbed on an oxide semiconductor electrode, for the purpose of improving energy conversion efficiency and the like.
In addition, Patent Document 2 discloses a cyanine dye that has a structure in which an indolenine skeleton is bonded to both ends of a heptamethine chain skeleton into which a cyclo-ring is introduced, and further has a carboxylic acid group as an anchor group to be adsorbed on an oxide semiconductor electrode, for the purpose of improving photoelectric conversion characteristics in the near infrared to infrared region.